The present invention relates generally to an improved method for acquiring magnetic resonance images (MRI), and more particularly, to a method and apparatus to acquire high temporal resolution MR images that is particularly useful in cardiovascular MR examinations.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or longitudinal magnetization, MZ, may be rotated, or tipped, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Moving objects are particularly difficult to image, especially if an imaging plane is set in space with the object moving in and out of the imaging plane. Such imaging is especially difficult when a second periodic motion is added thereto. For example, imaging of objects in a subject which is breathing causes a periodic motion of internal structures, which is also further complicated by the beating motion of the heart if the structure is near the heart.
Cardiovascular disease is the leading cause of morbidity and mortality in most industrialized nations today. Until recently, cardiac MR imaging methods have been of limited clinical value for several reasons. First, such methods had a relatively long acquisition time relative to the cardiac cycle which resulted in cardiac motion blurring. Second, the long duration of the imaging scan requires patients to hold their breath for unreasonably long periods to avoid motion artifacts due to respiratory motion. With the advent of segmented k-space fast gradient recalled echo (fgre) based sequences and, more recently, echo-planar imaging-based sequences, cardiac MR imaging has become more commonplace.
Segmented k-space methods acquire data over several cardiac cycles in a single breath-held acquisition. Data is partitioned into several segments, with each segment acquired in successive R—R intervals. Within any given R—R interval, the same segment is repeatedly acquired at different time points within the R—R interval resulting in a movie of images covering the entire R—R interval, yet having a high temporal resolution. The development of interleaved echo planar imaging (also known as echo-train, or fast gradient recalled echo-train (fgret)) imaging methods has significantly reduced the required imaging scan times by permitting the collection of multiple k-space lines from each RF excitation (ETL). Roughly, either the acquisition time can be reduced by a factor of the ETL, thereby maintaining the same temporal resolution, or the temporal resolution can be increased by a factor of the ETL, thereby maintaining the same scan time. Typical breath-holding times are 12 to 16 seconds for fgre-based acquisitions and 6-8 seconds for fgret-based acquisitions. The combined use of echo-train techniques with the segmented k-space acquisition allows for cardiovascular examinations to be performed using breath-hold techniques.
An example of such an examination is the MR-based exercise or pharmacologically-induced stress function examination which is an MR version of an electrocardiograph (ECG) based stress test. In stress function imaging studies, the patient is subjected to successively increasing levels of cardiac stress and once the heart rate has stabilized at the required stress level, MR images are obtained typically using breath-held, segmented k-space techniques.
However, during the transition time between the successive stress levels there is a need to continuously monitor the patient for any abnormal cardiac function such as cardiac wall motion changes or ischemic-related cardiac events. While monitoring can be performed using the aforementioned breath-held technique, repeated breath-holding can be very exhausting for this class of patients. Therefore, there is a need for a fast MR acquisition method that can be used for monitoring during the transition periods between the successive stress levels. Such a method must be able to acquire free-breathing images with sufficient spatial and temporal resolution to detect cardiac wall motion abnormalities related to ischemic events in near real-time. The high temporal resolution requirements is particularly important during a stress test when the cardiac cycle (R—R interval time) is significantly reduced due to the high heart rates encountered at higher stress levels. Typically, approximately 10 images per R—R interval are adequate to visualize the systolic phase. The spatial resolution requirement is approximately 3 mm, or better to detect wall motion abnormalities.
ECG-gating poses yet another obstacle. Such gating suffers from a number of problems such as operator dependence, inter-patient variability, detached ECG leads, and corruption of ECG signals due to noise from the imaging gradients. In situations where ECG-gating is problematic, one alternative is to use peripheral gating which is less complex and often more reliable than ECG-gating. However, peripheral gating suffers from the limitation of introducing a delay relative to the ECG-gating, so it is not possible to identify the exact moment of the cardiac R-wave. More generally, it would be desirable to have a technique which avoids having to detect any gating triggers in real-time.
One potential solution to this problem is MR fluoroscopy which employs an ungated fast imaging sequence, such as an interleaved EPI fgret, to acquire, reconstruct, and display data in real-time. Although MR fluoroscopy appears to be an appealing solution, computational requirements for rapid image reconstruction and display, together with spatial resolution requirements, restricts the maximum achievable frame rates to 12-15 frames per second which result in a temporal resolution of 66-85 ms. Such resolutions are unacceptable for heart rates of the order of 150-180 beats per minute (bpm) to effectively visualize the systolic phase. Another possible solution is to use a segmented k-space CINE sequence with a small views-per-segment (VSP) value. However, this still poses a problem in that it requires a large number of heartbeats to complete an acquisition, during which breathing artifacts can be significant. Therefore, this is also not a desirable solution to the problem.